cannot compile because cannot override mutable variable

override.scala

// cannot compile because cannot override mutable variable
// purpose of this pattern is to reuse logic from Base in Sub that works with BaseElements while Sub contains logic that works with SubElements
// not allowing override of vals has valid reasons so I'm looking for other options on how to design this

class BaseElement {
  def foo() { println("I like foo") }
}

class SubElement extends BaseElement {
  def bar() { println("I like bar") }
}

trait Base {
  var map = Map[Int, BaseElement]()

  // want to provide default impl for Sub (without depending on SubElement)
  map.get(0).get.foo();

  // want to be able to mutate Base (Map is immutable for thread safety so need to replace it)
  map = map -- map.keys.filter(_>0)
}

class Sub extends Base {
  override var map = Map[Int, SubElement]()
  
  // want to be able to call methods on SubElement without having to upcast
  map.get(0).get.bar();
}

One ugly workaround would be to just skip the override and cast BaseElement to SubElement... but hey, it's ugly so it doesn't count.